One of items to be corrected by a spectacle lens includes “fixation disparity”. Fixation disparity refers to a state where, when both eyes simultaneously look at (fixate) an object, one of or both of the eyes cannot form an image at a position of the central fovea of retina and the image is formed at a position slightly shifted from the central fovea. Fixation disparity occurs due to a shift in a sight line. Therefore, when a person with fixation disparity sees the same object by binocular vision, a position of forming an image when the object is seen with the left eye and a position of forming an image when the object is seen with the right eye are slightly shifted from the central fovea of the left and the right eyes, respectively. However, the shift in positions of forming images due to fixation disparity is quite small such as several minutes in the sight line angle and thus fusion of images in the brain results in perception of the images as one image. Therefore, the image of the right eye and the image of the left eye are not perceived as shifted in a binocular vision and a blur of images is rarely recognized. However, since the position of forming the image of each of the left and the right eyes is shifted from the central fovea thereof, a vision with the both eyes is deteriorated and a vision with the both eyes becomes unstable. However, this is rarely recognized by the person.
Moreover, a person having fixation disparity often feels physical pain such as fatigue of the eyes, headache, or shoulder stiffness. Checking a cause of the above by testing at a hospital or the like may clarify that the person has fixation disparity. In this case, in order to relieve the pain caused by fixation disparity, it is desirable to measure a prism amount, where a given prism exactly eliminates the fixation disparity, using a plurality of test lenses having different prism powers and to wear a spectacle lens prescribed a prism (prism correction) in accordance with this prism amount. As for the prism amount, a technique described in Patent Literature 1 is known for example. In this manner, the prism amount where the fixation disparity of the spectacle wearer is exactly eliminated is called an aligning prism.